Tunable laser absorption spectroscopy (TLAS) refers to techniques and methods used for detecting and measuring certain gas concentrations with the use of tunable diode lasers. TLAS utilizes absorption of laser emissions and absorption spectrometry to measure the concentration of certain gases in a gas mixture. The exact wavelength of the laser emission coincides with the absorption of molecular transitions of the constituent molecules of the specific gas. Together with techniques such as wavelength modulation spectroscopy, TLAS with simple absorption cells is capable of detection limits on the order of parts per million, which is sufficient for many applications of interest. The mid-infrared region from 3 to 5 μm presents particular promise since it contains several strong absorption features of scientifically important species in atmospheric science and environment monitoring, such as hydrogen chloride, water, methane, ethane, and other alkanes. Several laser sources are suitable for these techniques. In particular, interband cascade lasers (ICLs) based on the GaSb material system are well suited for high-power single-mode emission in the 3 to 5 μm wavelength range.
A common method of realizing monolithic, single-mode operation of ICLs is to incorporate a distributed feedback (DFB) grating. As typical buried-grating designs are impractical for GaSb-based devices due to issues with overgrowth in the material system, the Bragg grating must be incorporated into the design through different approaches. For instance, in some designs, a metal Bragg grating can be deposited on top of a ridge waveguide (RWG). Other designs are based on etching vertical corrugations in the sidewalls of the RWG. These corrugations can not only serve as a Bragg grating, but can also suppress higher-order lateral optical modes by introducing scattering losses at the edges of the RWG.